Photometry

It will be seen that for the measurement of either brightness or reflection factor a knowledge of p, the reflection factor of the photometer test surface, is necessary. This may be obtained by setting up the surface at one end of a photometer bench, illumi nating it with a lamp of known high candle-power at a known distance, and measuring the candle-power I of the surface by means of a photometer head and comparison lamp in the ordinary way. If E be the illumination of the test surface in foot-candles and s its area in square feet, I -=--pEs/r, so that p is found.

Heterochromatic Photometry.

No mention has so far been made of what is, perhaps, the greatest difficulty in all photometric measurement, viz., a difference of colour between the lights to be compared. The modern electric lamp gives light which is much "whiter," i.e., contains a larger proportion of blue, than that given by the carbon filament lamp.

Since the primary photometric standards are carbon filament electric lamps, operating at an efficiency of about 4.8 watts per candle, it is clear that, at some stage or another in the measure ment of modern light sources, large colour differences have to be bridged with as little loss of accuracy as possible. There are three generally recognized methods of doing this, viz., (i.) the cas cade method; (ii.) the flicker method, and (iii.) the colour filter method based on spectrophotometry. Each of these methods will be described briefly in turn, but mention must first be made of a peculiarity of the eye which complicates the problem. The curve shown in fig. i gives the relative response of the eye to equal amounts of energy at different parts of the spectrum. This curve, however, only applies when the brightness of the field of view is 0.15 candle per square foot or over. Below this limit the curve shifts gradually to the left so that at low brightnesses the maxi mum is at a wave-length of about 505 m/..t. It follows that, if two fields are illuminated, one by red and one by green light, it may quite well happen that, when the brightness of both fields is high, the red may appear the brighter of the two, while, if both be reduced in brightness in the same ratio, the balance may appear to shift over so that the green may appear brighter than the red. This effect is known as the Purkinje effect, and it has to be guarded against by ensuring that the brightness of the field of view is well in excess of the limiting value mentioned above.

In the cascade method of heterochromatic photometry, the colour difference between the two lights to be compared is divided into a number of small steps by the interposition of other sources giving lights of intermediate colour. The method, however, is not entirely satisfactory since, clearly, an observer who weights the "whiter" light, will do so throughout the series of comparisons, and so his errors at each step will add up in the final result.

The Flicker Photometer.

The flicker method depends on the fact that, when the field of view is illuminated alternately by lights which differ in colour and in intensity, flicker due to colour difference vanishes at a lower speed than flicker due to bright ness difference. A photometer head constructed on the flicker principle is designed so that the two surfaces which make up the comparison field are presented to the eye, not side by side, but one after the other in rapid alternation. At low speeds the field is observed to flicker whatever the position of the photom eter head, but as the speed is raised the flicker gradually decreases until finally it can be made to disappear at one position of the head only. If the speed be maintained at this critical value, a slight movement of the head from the position of balance causes a flicker to appear. The critical speed depends on the colour dif ference between the lights, being naturally higher the larger the difference. Any increase in speed beyond that just necessary to remove flicker at the point of balance results in a decrease of sensitivity. It is therefore a first essential of any flicker photom eter that it should be provided with an easy and rapid speed adjustment.

It is by no means self-evident that the results obtained by this method will agree with those obtained either by the cascade method or by a direct comparison involving the whole colour difference and carried out by a large number of observers using, say, the Lummer-Brodhun photometer. Much experimental work has been done on this subject and it has been found that the two methods are in agreement when the following conditions are com plied with in the flicker measurement : (a) a flicker field of about 2° diameter, and (b) a field brightness of about 8 candles per sq. metre. Since a 2° field is very small, and is most tiring for con tinuous work, the flicker field should be surrounded with a steady extended field of the same or slightly lower brightness.